Three problems from my time with the Surface Pro (admittedly only the first 3 versions, haven't played with an SP4 yet):
1) Apps that basically ignore touch-first with "meh, it works, and if not, they have a mouse". This tends to make the keyboard cover required rather than optional.
2) The virtual keyboard in Windows is just a horrible experience. It's like the Windows folks looked at the Windows Phone keyboard and said, "You know, that keyboard works fine, let's ignore it and learn nothing from it, and make a new one without understanding what makes a keyboard good or bad." This also tends to make the keyboard cover more than just an optional accessory.
3) The thickness and weight of the SP3, with the not-really-optional keyboard cover just make it unwieldy. I'd still take an iPad Pro over the Surface Pro if using it for tablet cases. It isn't that much worse, but it is still worse. The larger screen actually makes certain apps a bit more unwieldy as well (iPad Pro has the same issue, see Pocket which winds up using HUGE runs of text per line, making it less comfortable to read on).
The first two complaints are the big ones for me. An app experience which is not really touch friendly (and an app store that is basically barren of good touch-first apps), and a virtual keyboard that makes me want to re-attach the keyboard cover. The fact that I prefer a 10" tablet just puts the final nail in the coffin. (Oh, and the original Surface RT which decided to use a 16:9 resolution, turning it into a nice lever in your hands was awesome)
I actually have both right now trying them out. The surface is ok (not great) as a laptop. Really bad as a tablet. There's simply not much I like about any windows metro apps or whatever you want to call them. Going back to desktop apps (using as a tablet) just sucks. You simply need the kb attached for that.
2. The fan. Doing the simplest thing seems to run the fan and it gets hot. Battery is sinking before my eyes.
3. No problem with size or weight. The kickstand is awesome actually. Wish the ipad had one.
4. Convenience. Ipad apps take no time at all to get to and load up. Windows takes longer. Try Hearthstone or reading comics on the surface. It's just easier on the ipad and a much smoother experience. An ipad is enjoyable to use. A surface kinda isn't.
So, sure, the surface can do much more than the ipad. But only as a mediocre laptop. The surface also lags and stutters a bit. Had to do a hard reset tonight when it wouldn't wake up. I don't use the stylus much which is probably why most get a surface.
I'd rather have a great laptop and great tablet. That's an ipad and for now, a MBP. It could easily be a dell xps though. And I'll be honest. I've been a windows user since it saw light. And DOS before that. I HATED windows 8 (windows 10 is only slightly better). I hate the direction MS was going in. I hate they changed the defaults for music, video, etc to those silly metro apps. I hate the whole tiles UI. I still need a program like Start8 or 10 now to make start usable for me again. But yeah, damn, I used to first in line to get new windows or office versions. I teach Office classes, especially Excel and have done windows sessions for CPE credit for CPA's.
Windows 8 is what got me to buy a Mac. It didn't hurt I was an iphone user and Apple was bringing over iOS features to the Mac either. I can't really see switch back. I have windows on the Mac for what I need it for (and that's quite a few things) At work, I still use a windows laptop. But it's their laptop, not mine.
That all said, I'm not sure I want the ipad pro either. It's a big, mean, ipad. Killer screen and speakers. But maybe overkill for what I use an ipad for. I'm not much for a stylus. But yet I want to wait and try the Pencil I'll get this week.